Help with a Custom Planet Texture
Here is how I would solve this problem.



I would convert the texture into black and white using a color ramp in the node editor, where the water is black, and the land mass is grey or white (This is because white is the value pops out, black doesn't). You might need to make slight adjustments to the color ramp. (The noise texture in the demonstration should be your final planet texture)





Next, add a mix RGB node into the node editor. Also add a noise texture into the node editor. Plug the noise texture into the bottom of the mix RGB node, and set the top color of the mix RGB node to black. it is recommended to increase the detail and scale in the noise texture.

Last but not least, add a displacement node into the node editor and plug it into the displacement of the material output. Next, plug the mix RGB color into the height of the displacement node. You might need to change the scale of the displacement node. (The strength or "height" of the displacement)



I hope this works if you try it out. There are probably other ways to do this, but this is how I'd personally do it.